[coll] Implement a new tracker and a communicator. (#9650)

* [coll] Implement a new tracker and a communicator.

The new tracker and communicators communicate through the use of JSON documents. Along
with which, communicators are aware of each other.

tests/cpp/collective/test_worker.h

@@ -0,0 +1,91 @@
+/**
+ * Copyright 2023, XGBoost Contributors
+ */
+#include <gtest/gtest.h>
+
+#include <chrono>   // for seconds
+#include <cstdint>  // for int32_t
+#include <string>   // for string
+#include <thread>   // for thread
+#include <utility>  // for move
+#include <vector>   // for vector
+
+#include "../../../src/collective/comm.h"
+#include "../../../src/collective/tracker.h"  // for GetHostAddress
+#include "../helpers.h"                       // for FileExists
+
+namespace xgboost::collective {
+class WorkerForTest {
+  std::string tracker_host_;
+  std::int32_t tracker_port_;
+  std::int32_t world_size_;
+
+ protected:
+  std::int32_t retry_{1};
+  std::string task_id_;
+  RabitComm comm_;
+
+ public:
+  WorkerForTest(std::string host, std::int32_t port, std::chrono::seconds timeout,
+                std::int32_t world, std::int32_t rank)
+      : tracker_host_{std::move(host)},
+        tracker_port_{port},
+        world_size_{world},
+        task_id_{"t:" + std::to_string(rank)},
+        comm_{tracker_host_, tracker_port_, timeout, retry_, task_id_} {
+    CHECK_EQ(world_size_, comm_.World());
+  }
+  virtual ~WorkerForTest() = default;
+  auto& Comm() { return comm_; }
+
+  void LimitSockBuf(std::int32_t n_bytes) {
+    for (std::int32_t i = 0; i < comm_.World(); ++i) {
+      if (i != comm_.Rank()) {
+        ASSERT_TRUE(comm_.Chan(i)->Socket()->NonBlocking());
+        ASSERT_TRUE(comm_.Chan(i)->Socket()->SetBufSize(n_bytes).OK());
+      }
+    }
+  }
+};
+
+class SocketTest : public ::testing::Test {
+ protected:
+  std::string skip_msg_{"Skipping IPv6 test"};
+
+  bool SkipTest() {
+    std::string path{"/sys/module/ipv6/parameters/disable"};
+    if (FileExists(path)) {
+      std::ifstream fin(path);
+      if (!fin) {
+        return true;
+      }
+      std::string s_value;
+      fin >> s_value;
+      auto value = std::stoi(s_value);
+      if (value != 0) {
+        return true;
+      }
+    } else {
+      return true;
+    }
+    return false;
+  }
+
+ protected:
+  void SetUp() override { system::SocketStartup(); }
+  void TearDown() override { system::SocketFinalize(); }
+};
+
+class TrackerTest : public SocketTest {
+ public:
+  std::int32_t n_workers{2};
+  std::chrono::seconds timeout{1};
+  std::string host;
+
+  void SetUp() override {
+    SocketTest::SetUp();
+    auto rc = GetHostAddress(&host);
+    ASSERT_TRUE(rc.OK()) << rc.Report();
+  }
+};
+}  // namespace xgboost::collective